I have my power inverter in the trunk off a distribution block, the other connects to my amp. I had noise until I grounded the inverter (short distance to a bolt in the back) I also grounded the PC to this same spot and also the amp. I had noise if I hook anything up to my cig lighter so I decided to plug my lillput in the inverter. Once I grounded the PC case to this same stop, it all worked fine.
I then had to worry about flickering on my lilliput. I soldered a cable to the vga extension cable (anywhere you want on the outside of it) and grounded it too. No noise, no flickering. Make sure your grounds are good. Thats what helped me.